Acting as another user won’t give you the access permissions of
that user. A secretary who has authority to act as her boss in
order to manage his calendar should not also see e.g. statistic
reports to which she has no access. For system admins it is
different: when a system admin acts as another user, he inherits
this user’s access permissions. System admins use this feature to
test the permissions of other users.

The truth is: independently of whether you are admin or not, acting as
another user means that you get that other user’s menu.

show is now a bit more user friendly: uses None instead
of “root” as default username, and gives a meaningfule message when
the specified username does not exist.